Various engineered waveguides or structures with electromagnetic resonant characteristics have been studied for optical sensing applications, such as noninvasive refractive index monitoring. Many of the structures exhibit resonance responses, where a dip in transmission occurs at a characteristic resonant frequency. The resonant frequency can be substantially dependent on the refractive index of the surrounding medium, and consequently be used as a highly sensitive measure for changes in the refractive index. For example, planar integrated waveguide resonators and asymmetric split ring arrays have been used to detect Deoxyribonucleic acid (DNA) hybridization and denaturing. Additionally, coupled Terahertz (THz) resonators and resonant metal meshes have been studied for biomedical sensing, and planar structures have been used to study nanometer-thick films of material. However, most of the structures that have been studied have planar or open geometry, which is not compatible for flow monitoring in microfluidics platforms and on-line applications. Further, the resonant frequency linewidth of such structures limits the refractive index detection resolution, where sub-linewidth shifts in resonant frequency are difficult to detect.
Electromagnetic radiation at THz frequencies and sub-millimeter wavelengths have also being investigated for sensing applications. One of the waveguide structures that have been examined to transport the waves at THz frequencies is the parallel plate waveguide (PPWG), which comprises two parallel metal plates. The PPWG has been investigated for its promising wave propagation characteristics, such as relatively lower attenuation and distortion at THz frequencies and no low frequency cutoff. However, no effective PPWG integrated resonators have been successfully introduced.